Embodiments of the invention relate to field effect transistors such as MOSFET (metal oxide semiconductor field effect transistor) devices and methods for making field effect transistors.
Power MOSFET devices are well known and are used in many applications. Exemplary applications include automotive electronics, portable electronics, power supplies, and telecommunications. One important electrical characteristic of a power MOSFET device is its drain-to-source on-state resistance (RDS(on)), which is defined as the total resistance encountered by a drain current. RDS(on) is proportional to the amount of power consumed while the MOSFET device is on. In a vertical power MOSFET device, this total resistance is composed of several resistive components including an inversion channel resistance (“channel resistance”), a starting substrate resistance, an epitaxial portion resistance and other resistances. The epitaxial portion is typically in the form of a layer and may be referred to as an “epilayer”. RDS(on) can be reduced in a MOSFET device by reducing the resistance of one or more of these MOSFET device components.
Reducing RDS(on) is desirable. For example, reducing RDS(on) for a MOSFET device reduces its power consumption and also cuts down on wasteful heat dissipation. The reduction of RDS(on) for a MOSFET device preferably takes place without detrimentally impacting other MOSFET characteristics such as the maximum breakdown voltage (BVDSS) of the device. At the maximum breakdown voltage, a reverse-biased epilayer/well diode in a MOSFET breaks down resulting in significant and uncontrolled current flowing between the source and drain.
It is also desirable to maximize the breakdown voltage for a MOSFET device without increasing RDS(on). The breakdown voltage for a MOSFET device can be increased, for example, by increasing the resistivity of the epilayer or increasing the thickness of the epilayer. However, increasing the epilayer thickness or the epilayer resistivity undesirably increases RDS(on).
It would be desirable to provide for a MOSFET device with a high breakdown voltage and a low RDS(on). Embodiments of the invention address this and other problems.